This invention generally relates to a process of subjecting low moisture content cellulose pulp to mechanical treatment which gives rise to structural deformation of the fibers, causing them to become convoluted, i.e., twisted and bent in a substantially lasting manner, without appreciably reducing the fiber length and without substantially decreasing the freeness of the pulp. At the same time in this process, the pulp is fiberized and fluffed so that the interfiber bonds between individual fibers, e.g., fiber bundles, which typically are created in drying the pulp are to a great extent broken and substantial disentanglement of the fibers results.
The prior art describes a number of methods of subjecting cellulose pulp to mechanical treatment for modifying the structure and configuration of the fibers by working a mass of fibers in a confined space between working elements. The conditions under which such methods are conducted differ considerably but, commonly, the pulp to be treated is in a substantially wet condition. By reason of the moisture content of the pulp so treated and the other operating conditions employed by the prior art processes, the degree and character of structural deformation which can be imparted to the fibers is limited. Moreover, these prior art processes do not at the same time fiberize and fluff the pulp to any significant degree. Indeed, these processes frequently tend to further entangle the individual fibers so that a separate fiberizing step is required.
One such prior art process of this type employs the so-called "Curlator" machine and is described in U.S. Pat. 2,516,384 to Hill et al. Other types of specific equipment for carrying out this process are described in U.S. Pat. No. 2,561,013 and 3,028,632, both to Coghill. In the Hill et al. process, cellulose fibers are "curled" to produce some degree of kinking, bending and twisting of the individual fibers. As opposed to conventional refining methods, this curling treatment does not substantially change the freeness of the pulp, but the tensile and bursting strengths decline as the stretch and tearing strengths, porosity and softness increase. In this process, cellulose pulp at a consistency between 2% and 60% is confined under mechanical pressure between two elements which are in relative gyratory or reciprocal motion creating nodules or balls of pulp between the opposed working elements. This gyratory action of the elements on the compressed nodules, which is quite different in nature and involves a generally less drastic application of forces than, for example, conventional refining, imparts the above described kinks, bends and twists to the pulp fibers. Thus, due to the limitations imposed by the need for gyratory or reciprocal motion, as well as the relatively high moisture content of the pulp, this process is inherently limited in through-put capacity. Moreover, the degree of fiber deformation is relatively low in the Hill et al. process and is such that any amount of convolution imparted to the fibers is quite limited. Thus, the fiber modification imparted by Hill et al. is not lasting in nature since an appreciable amount of the twists, kinks and bends transmitted to the fibers is dissipated on standing in about a 24- to 48-hour time period. Thus, deformation of the fibers in the Hill et al. process is mainly plastic in nature, the fibers tending to revert to their original configuration with time. This is believed to be at least partially due to the substantial amount of water that surrounds and is contained within the fibers, which tends to reduce the amount of lasting structural distortion which might otherwise result. Moreover, the fibers of the so-treated pulp are interlocked and intertwined so that a separate process step is required in order to fiberize the pulp as, for example, described in U.S. Pat. No. 3,809,604 to Estes.
Another such process, the so-called "high consistency refining", is described in U.S. Pat. No. 3,382,140 to Henderson et al. This process has the purpose of refining fibers by interfiber friction in order to increase tensile and burst strengths without decreasing tear strength. In this process, pulp at a consistency between 10% and 60% by weight is fed into a working space between two opposed, relatively rotating discs which confine the pulp therebetween under a pressure of from 5 to 20 pounds per square inch. The relative movement of the disc surfaces creates interfiber friction in the mass of confined fibers. The amount of work imparted to the fibers is quite substantial as measured by the energy input to the refiner, which may be as high as 60 horsepower days per ton. This interfiber frictional treatment refines the fibers, i.e., their surfaces are fibrillated and the tensile and burst strengths are substantially increased as the freeness of the pulp correspondingly decreases. This treatment also tends to kink and twist the fibers but such deformation is necessarily accompanied by fibrillation of the fibers, lowered freeness, etc., as previously mentioned. Moreover, the fibers of the treated pulp are interlocked and intertwined by the process such that a subsequent step is necessary to fiberize the pulp.
As mentioned, there are also various prior art processes for fiberizing and fluffing substantially dry pulp, the fibers of which are intertwined and bonded together. Thus, processes utilizing hammermills, pinmills or disc refiners may be employed for the purpose of separating an intertwined dry pulp mass into individual fluffed fibers having minimal reductions in fiber length and freeness. In general, these processes exhibit only a minor amount of fiber deformation. Typically, therefore, little actual work is imparted to the pulp in conducting these processes.
Processes in which a disc refiner is employed for purposes of pulp fiberizing are described in U.S. Pat. No. 3,596,840 to Blomqvist et al. and U.S. Pat. No. 3,802,630 to Lee et al. In these processes, dry pulp (in Blomqvist pulp with a consistency greater than 85%, and in Lee et al. at least 90%) is introduced into a disc refiner operated under conditions which will fiberize and fluff the pulp. In the case of Blomqvist et al. a fixed gap width is maintained between the refiner plates of between 0.1 to 5 mm and the pulp is fed into and through the gap entrained in a carrier gas stream. The fibers are separated by a rubbing or shearing action of the plates on the pulp. No information is provided as to the amount of work imparted to the fibers. However, it is apparent that little work or resulting fiber deformation is carried out since the fibers pass rapidly through the gap entrained in a gas stream and thus do not fill the work space to the extent that will permit exertion of sufficient pressure and accompanying forces on the pulp by the refiner plates to create fiber deformation. Similarly, in the process described in Lee et al. the operating conditions maintained between the plates are insufficient to permit the space between the plates to be filled with fibers under the requisite compression. Consequently, while interaction of the pulp with the refiner plates may be sufficient to fiberize the pulp, forces of a character and degree to twist and bend the fibers in a substantially lasting manner are not generated.
U.S. Pat. No. 3,301,746 to Sanford et al.; U.S. Pat. No. 3,432,936 to Cole et al.; and U.S. 3,821,068 to Shaw, all provide methods of forming soft, absorbent, bulky sheets employing techniques in which compaction of a wet web, prior to drying, is omitted since it is totally detrimental to proper sheet formation.